Many communication appliances, as well as recording equipment, have keyboards for control in the same physical enclosure as a microphone capturing the sound. Examples of such appliances are desktop videoconferencing equipment, audio conference phones, mobile telephones, other types of telephones, MP3 recorders, tape recorders or similar devices.
When pressing a key on the keyboard, an acoustic sound (keyboard noise) is created. Normally, this sound is unwanted. In the local room, the keyboard noise is usually of such low level that it doesn't disturb the user very much. When the audio is captured at a microphone, for communication or archiving, keyboard noise may be disturbing. Noise from the keyboard may transfer to the microphone both as sound propagating through the air, and as sound propagating through the physical structure of an equipment enclosure.
Keyboard noise usually lasts for a very short time, but is often picked up relatively strong at the microphone, and it is usually broadband noise. Such keyboard noise has previously been handled in various ways. The simplest one is accepting the noise. Another previous approach is known as masking, wherein a masking tone is added at the same time as the occurrence of the keyboard noise Such masking may make the keyboard noise inaudible or neglectable, or it may make the user focus on something else, or at least make the user aware that something is happening and therefore more accepting for the audible noise. Another suggested approach is muting the entire audio signal (including both the keyboard noise and the wanted signal) when a key is pressed on the keyboard.
Spectral subtraction is a widely used approach for removal of relatively low level, stationary broadband noise. Based on an assumption that noise is stationary, it calculates an estimate of the noise, from which it defines a linear time invariant filter, which is applied to the wanted signal including unwanted noise. The result is an output signal with a magnitude spectrum which is fairly equal to the magnitude spectrum of the wanted signal, but with a phase equalling the wanted signal plus unwanted noise phase. As long as the noise level is moderate, the phase error is small, yielding a well sounding result. When noise level increases, artifacts become more and more audible and annoying. People skilled in the art will consider spectral subtraction useful for stationary noise, not for transient noise such as keyboard noise.